Silicon is a chemical element sometimes used together with hard material layers to increase the layer stress. Increasing the layer stress usually leads to an increase in the hardness of the layer. This is used, for example, also in connection with titanium nitride. The resulting layers can be chemically described by the structural formula TixSi1-xN, wherein x is the concentration of Ti expressed in at % when only metallic elements are taken into consideration. When written in this way, the atomic concentrations indicated in percent add up to 100%.
Such layers can be produced in extremely hard form by means of the so-called cathodic spark evaporation. Herein, a spark is ignited between a target providing the metallic elements, which is used as the cathode, and an anode, by means of which a high-density electron stream is extracted from the target surface. Due to the extremely localized, very high current density at the target surface, the target surface is locally overheated and the material evaporates in ionized form.
The material thus evaporated in ionized form is then accelerated toward the substrates with the aid of a negative voltage applied to the substrates. If a reactive gas is additionally introduced into the coating chamber, the evaporated ions combine with the reactive was and form a corresponding coating on the substrate surface.
However, in this method the so-called droplet problem can frequently be observed: the sudden local heating on the target surface causes an explosive melting to occur, by which whole drops of the target material are propelled into the surrounding area. Some of these drops then come to lie on the substrate surface generally resulting in negative effects on the layer properties and their quality. While there are meanwhile methods to filter out these droplets, such filters dramatically reduce the coating rate, and it becomes virtually impossible to operate the coating process economically.
On the other hand, a silicon content of greater than 15 at % very often leads to damage to the target during spark evaporation. In extreme cases, the target has to be exchanged after each coating process, again reducing the economic viability of the process.
In the case of conventional deposition from the gas phase by means of evaporation with magnetron support (magnetron sputtering) the person skilled in the art does not have to contend with these problems. However, the particles chipped out of the target surface by means of ion bombardment, are not ionized or ionized to a very limited extent, and can thus not be accelerated toward the substrates by means of a substrate bias applied to the substrates. As a consequence, the layers sputtered in the conventional manner have relatively low density and hardness.
A well-known approach to achieve densities and hardnesses of sputtered layers similar to those achieved with the spark evaporation method, is the so-called HiPIMS method (HiPIMS=high power impulse magnetron sputtering). In this sputtering method, high power pulse densities are applied to a sputtering cathode, which results in the material sputtered by the cathode being ionized to a high percentage. If a negative voltage is now applied to the workpieces to be coated, these ions are accelerated towards the workpieces leading to very high-density layers.
The power applied to the sputtering cathode must be applied in a pulsed fashion to give it time to dissipate the heat introduced together with the power. In the HiPIMS method a pulse generator is thus needed as the power source. This pulse generator has to be capable of outputting very high power pulses, which are, however, very short. The pulse generators available today have little flexibility for example with regard to the pulse height and/or pulse duration. Ideally, a square-wave pulse should be output. However, the power output within a pulse is generally highly dependent on time, which has an immediate effect on the layer properties, such as hardness, adhesion, residual stress et cetera. Moreover, the coating rate is negatively affected by the deviation from the square wave profile.
These problems, in particular, raise questions with respect to the reproducibility.
As far as the inventors know, attempts have not yet been made to produce TixSi1-xN layers by means of the HiPIMS method.
There is therefore a need fix a method according to which TixSi1-xN layers can be produced by means of magnetron sputtering with high power.